Alcohols are a general class of flammable organic compounds having appreciable solubility in both aqueous (hydrophilic) and organic (hydrophobic) solvents. As a result, alcohol purification processes usually include a water-removal step. Effective water-removal steps can be expensive, energy-intensive, or otherwise inefficient, depending in part upon the alcohol involved and the degree of water-removal required.
Various lower alcohols, including methyl, ethyl and butyl alcohols, have been tried for use as a motor fuel. Use of a lower (C.sub.1 -C.sub.5) alcohol in a blended fuel for modern internal combustion engines usually necessitates the removal of virtually all water from the alcohol used to make the alcohol-fuel blend.
Ethyl alcohol, also termed ethanol, is generally regarded as the alcohol of choice for use in formulating fuel blends. It has been known for many years that ethanol has appreciable anti-knock properties when used in a fuel mixture for internal combustion engines. In the past, ethanol was occasionally mixed with gasoline to eliminate the "knocking" experienced when only gasoline was used as a fuel. Ethanol also appears to have a synergistic affect when added to gasoline which reduces polluting emissions from the engine. As a result, several states are considering mandating use of ethanol blends as motor fuels. Such blends are popularly termed "gasohol". Gasohol typically comprises blends containing as much as 10% (v/v) ethanol in gasoline.
Unfortunately, early ethanol-fortified gasolines contained too much water. Because it is impossible to remove all the water from ethanol by simple distillation, early mixtures that were formulated by mixing "distilled" ethanol with gasoline were usually saturated with water, even at temperatures greater than normal engine operating temperature. During cooler engine operating conditions, the fuel mixture sometimes formed a separate liquid water phase which impaired engine operation. Modern engines designed for tighter motor fuel standards are simply incapable of operating with such water-containing fuel mixtures.
Modern gasohol, which contains practically no water, has not yet achieved widespread favor as a motor fuel because it is too expensive to produce relative to the prevailing costs of producing gasoline. The principal reason for such high costs is the relatively large amount of energy required to produce ethanol with sufficient yield, purity and absence of water to add to gasoline. Nevertheless, gasohol remains a viable alternative to gasoline because the ethanol fraction is a renewable resource (via fermentation) that potentially can decrease the present rate at which nonrenewable petroleum reserves are being consumed.
Much of the attention directed at producing gasohol and other liquid alternative transportation fuels from renewable sources has centered on processes that begin with fermentation. Fermentation is a process in which dilute aqueous solutions of sugars, starches readily convertible to sugars, or agricultural "waste" products containing sugars or starches are inoculated with special strains of yeast or bacteria which enzymatically convert those nutrients to carbon dioxide and ethanol. The maximum achievable concentration of ethanol in a fermentation beer is approximately 15%, with continuous processes typically producing lower concentrations. Higher ethanol concentrations from fermentation alone are not possible because an ethanol concentration greater than approximately 15% (w/w) causes death of the fermentation microorganisms. Such a low concentration of ethanol in water cannot be used as a motor fuel or fuel additive because it contains too much water. The ethanol must first be concentrated and dehydrated.
One type of gasohol production process in current use employs, after fermentation and clarification of the fermentation beer, a subsequent two-step distillation to yield "absolute", or 100%, anhydrous ethanol that can be blended with gasoline. The first distillation yields an azeotrope containing distillation is an azeotropic distillation, in which a third compound such as benzene is added to "break" the azeotrope and produce 100% ethanol. While this process effectively separates ethanol from water, it is unfortunately energy-intensive. The two distillation steps together account for more than 50% of the energy required for production of absolute ethanol from fermentation. If that energy requirement could be substantially lowered, the cost of gasohol production from fermentation would decrease and the product more widely accepted as a serious alternative to gasoline for use as a motor fuel.
Another process sometimes used for dehydration of dilute solutions of ethanol or other lower alcohol is solvent extraction, where the aqueous solution of the alcohol is mixed with a liquid organic solvent having an affinity for alcohol greater than the affinity of the alcohol for water, thereby causing a net transfer of the alcohol from the water to the organic solvent. The principal advantage of solvent extraction is that very little energy is consumed. However, in the case of ethanol, for example, no known organic solvent has an affinity for ethanol at ambient temperature that is appreciably greater than that of water. Consequently, such processes at ambient temperature require large volumes of solvent. There is also the problem of separating the ethanol from the solvent used to perform the extraction.
U.S. Pat. No. 4,490,153 (Sze and Suziu) disclosed a process by which dilute aqueous ethanol from a clarified fermentation beer is distilled ("rectified"), yielding a 90% (w/w) aqueous solution of ethanol which is extracted at low temperature (+5 to -10.degree. F.) with a large excess volume of gasoline. The low-temperature extraction produces a gasohol blend containing approximately 10% (w/w) ethanol in gasoline and a separable liquid aqueous phase containing a small amount of ethanol and traces of gasoline. Unfortunately, that process consumes large amounts of energy in the initial distillation step where most of the water is removed. Hence, the 4,490,153 patent does not satisfy the need for a gasohol production process that consumes low amounts of energy.
Another technique is "freeze concentration" in which a solution is cooled to below its freezing point, forcing some of the solvent to freeze and increasing the concentrations of solutes remaining in the liquid phase. In U.S. Pat. No. 4,385,944, Hewitt and Tillen disclosed such a process in which a dilute aqueous solution of ethanol is added dropwise to a liquid hydrocarbon or dense gas used as a "heat-exchange liquid" at successively lower temperatures at which some of the water freezes, producing ice crystals and an ethanol-enriched liquid. The selected hydrocarbon is one that will not mix with the ethanol solution. Consequently, the ethanol-enriched solution can be separated from the hydrocarbon and from the ice crystals. However, the ethanol-enriched solution still has too much water (8 percent or more), even after multiple cycles through the Hewitt and Tillen process, to be useful as a motor fuel. No purification by solvent extraction or other process is disclosed.
Another nondistillation method of dehydrating aqueous ethanol solutions employs a solid dehydration agent or adsorbent to remove the water from the solution. The dehydration agent may be either discarded or regenerated after adsorbing the water. Examples of such adsorbents include alumina, zeolite, bauxite, fuller's earth, and acid-activated bentonite. In dehydration processes, it is usually necessary to first distill the fermentation beer to yield a distillate containing 70 to 95% (w/w) ethanol. Afterward, the distillate is contacted with a dehydration agent, producing an essentially anhydrous grade ethanol. Although the energy required for dehydration alone is less than that required for distillation, large scale production of absolute ethanol via a dehydration process usually requires a first stage distillation so that the dehydration agent does not need to remove such large volumes of water. Consequently, the dehydration method does not yield a significant energy savings over distillation.
Accordingly, there is a need for an efficient process that will extract lower alcohols from dilute aqueous solutions using a minimum amount of energy. There is also a need for an efficient method for producing an alcohol blend suitable for use as a high octane additive for motor fuel. There is also a need for such a process that is continuous. There is also a need for such a continuous process that transfers ethanol from dilute solutions formed by fermentation of renewable resources into a suitable organic solvent, the resulting solution being useful as a motor fuel additive, and the process being one that requires minimum energy and a minimal volume of extracting solvent for high recovery of ethanol from the dilute solution.